I already a lot of changes in conjunction with the support. Here is a list of the changes:

Change the vCPU and vRAM resources
Installed a proxy without any other program just dedicate it to VBO 365
Moved the Threads to 64.
Disabled the network bandwidth
Configured the exclusions on my antivirus

Even after doing these changes my rates are really low even worst than before doing the changes.

I am trying a full backup as requested from the support and now is not able to complete, the rates backing up SharePoint are dropping to zero randomly indicated that the bottleneck is the "Source" and sometime the "Target".

Also the last job stopped with errors and warnings two times since i started the backup job.

I don't know what else can it be and also i don't know why the rates go to zero randomly.

Am I correct that you're experiencing these issues only when backing up SharePoint sites?
Is the performance not sufficient for full backups or for incremental runs?
Which specifically errors/warnings did you get?

But i also experienced this with OneDrive, maybe is it because of the Microsoft Throttling?

I am not having erros neither warnings. Before doing the test of a new backup full (currently running) needed from the Veeam Support i was able to do a full without errors but the incremental of the SharePoint site never finished.

There can be a variety of reasons for slow SharePoint/OneDrive backups and throttling is probably the most common one (others may include some files blocked on the source side, lists with over 5K files in them, etc.). Please continue working with the support team to identify the root cause based on your logs; and I'll also track the progress of the investigation by your case ID.